1. Field of the Invention
The field of the invention is that of the transmission of modulated signals having a non-constant modulation envelope. To be more precise, the invention concerns the predistortion of such signals before they are transmitted.
2. Description of the Prior Art
Thus the invention applies in particular to QPSK or QAM modulation. The invention has been particularly developed for the TETRA (Trans European Trunk Radio System) standard for private mobile radio (PMR), intended, for example, for the networks used by the fire and police services, taxi fleets, etc.
The TETRA system uses .pi./4-DQPSK modulation in the base transceiver station (BTS).
Non-constant envelope modulation imposes operation over a wide range of the dynamic range of the amplifier (unlike constant envelope modulation). It therefore imposes a great variation of the power response of the amplifier, which leads to distortion phenomena.
This problem is well known. Predistortion of the signal before it is amplified is used to combat it. This predistortion must be calculated for optimum compensation of the distortion induced by the amplifier, so that the resultant signal is degraded as little as possible by the distortion.
Various predistortion techniques are already known in themselves. These include an analogue method known as cartesian co-ordinate feedback and a self-adaptive method.
The predistortion to be applied can be calculated by the spline method. This technique is described in "A practical guide to splines", C. de Boor, Springer Verlag, 1978.
The object of this technique is to determine the AM--AM (amplitude) and AM-PM (phase) characteristics of the amplifier in order to control the predistortion to be applied.
To this end, a reference signal is transmitted to the amplifier (a two-tone signal in the TETRA case), the response of the amplifier is digitized to produce a series of samples and the AM--AM and AM-PM characteristics are modified using the spline technique.
This modeling consists of the production of a curve by linking two successive samples by cubic interpolation. In other words, a sliding window selecting four consecutive points is applied to the samples and, for the two central points, the polynomial defining the curve passing through these four points is found and its second derivative is minimized.
This technique has several drawbacks.
In particular, the modeling obtained in this way is relatively sensitive to quadrature and balance defects and to the intermediate frequency (IF) phase noise.
The modeling curve obtained (see FIG. 2, discussed below) is generally very "agitated" in the presence of measurement errors and therefore passes through all the samples produced.
Objectives of the invention include alleviating these various drawbacks of the prior art.
To be more precise, one objective of the invention is to provide a method of modeling the AM--AM and AM-PM characteristics of an amplifier.
Another objective of the invention is to provide a method of this kind producing smoother calculated curves than the conventional spline technique.
A further objective of the invention is to provide a method of this kind reducing spurious transmission in adjacent channels and in particular, in the case of the TETRA system, rejection better than is required by the standard.
An additional objective of the invention is to provide a method of this kind with limited sensitivity to:
quadrature defects, PA1 balance defects, PA1 IF phase noise, PA1 noise of any type. PA1 transmitting a reference signal to the amplifier, PA1 digitizing the response of the amplifier to the reference signal to produce two sets of AM--AM and AM-PM reference samples, respectively, PA1 determining two series of polynomials respectively representative of the AM--AM and the AM-PM characteristics from the samples allowing for each of the polynomials for the second derivative of the polynomial and for the distances between the samples and points of the curve defined by the polynomial. PA1 second derivative of the polynomial, PA1 distance between the polynomial and one sample, PA1 distance between the polynomial and the other sample. PA1 the quadrature error, PA1 the balance error, PA1 the temperature, PA1 the defects induced by the demodulator, PA1 the phase noise, PA1 the overall noise, PA1 the average characteristic of the curve. PA1 means for digitizing the response of the amplifier to the reference signal delivering two sets of AM--AM and AM-PM reference samples, respectively and PA1 means for determining two series of polynomials respectively representative of the AM--AM and the AM-PM characteristics from the samples allowing for each polynomial for the second derivative of the polynomial and for the distances between the samples and the points of the curve defined by the polynomial. PA1 transmitting a reference signal at regular intervals to the amplifier, PA1 digitizing the response of the amplifier to the reference signal so as to produce two sets of AM--AM and AM-PM reference samples, respectively, PA1 determining two series of polynomials respectively representative of the AM--AM and the AM-PM characteristics from the samples allowing for the second derivative of the polynomial and for the distances between the samples and points of the curve defined by the polynomial, PA1 measuring a parameter representative of the signal to be transmitted, and PA1 predistortion multiplication of the signal to be transmitted in accordance with the polynomials and the representative parameter. PA1 the power of the signal to be transmitted, PA1 the square of the power of the signal to be transmitted, PA1 the modulus of the signal to be transmitted.
Another objective of the invention is to provide a method of the above kind that can replace the spline technique without requiring any modification of the transmitter.